18Watt.com

I would like to test a bunch of salvaged preamp tubes, in an actual amp at nominal operating points.

I was thinking of using a standard preamp (the normal channel of a Lite2b), and adding a 100 ohm resistor, in series, after the blocking cap on each plate.

If I insert a signal into the grid, of a known value (100mv) and measure the voltage swing across the resistor, can I calculate Gm somewhat reliably?

Suppose it could be done, I'm contemplating building a dedicated grid shift tester for 12AX7 as my B&K 700 thought great for testing shorts and grid emissions is lacking testing those dual triodes gm accurately..

David

http://www.antiqueradios.com/forums/vie ... hp?t=61785

Just measure the output signal at the plate. First put in a few known good ones to get a sense of what is normal, than measure away. The important thing is that the cathode is bypassed (so the tube itself is the limiting factor) and that the load (what's on the other side of the anode coupling cap, if using an existing amp) does not change.

If you want to know the actual transconductance, a little math will tell you.

--mark h